1. Field of the Invention
The present invention relates to an image exposure apparatus and, more particularly, to an image exposure apparatus that scans a photosensitive body with an irradiating light beam.
2. Description of the Prior Art
FIG. 9 shows a typical prior art image exposure apparatus that scans a photosensitive body by irradiating the photosensitive body with a light beam. In FIG. 9, a rotating drum 6 is driven by a drum driving unit 61 that uses an input clock signal as its synchronizing signal (SYNC). The rotating drum 6 rotates at a constant speed. Photosensitive paper 7 is wrapped around the surface of the rotating drum 6. A scanner 4 comprises: a motor 43 that rotates at a predetermined rotating speed; a rotating polygon mirror 41 rotated in an arrowed direction A by the motor 43; and an f-.theta. lens 42 for converting a laser beam moving at an isometric speed into a beam that moves at a constant speed over the surface B of the photosensitive paper 7. A laser beam generator 3 comprises a driving circuit 31, a semiconductor laser device 32 and a collimator lens 33.
The driving circuit 31 drives the semiconductor laser device 32 which then emits a laser beam. The laser beam emitted by the semiconductor laser device 32 is converted into a beam of parallel rays by the collimator lens 33. A mirror 51, located close to the rotating drum 6, causes the laser beam to enter a beam detector 5 before the beam scans the photosensitive paper 7 for exposure.
The control timings of the above prior art image exposure apparatus are provided as follows. The rotating polygon mirror 41 and the rotating drum 6 operate on a reference clock signal output by an oscillating circuit 8. The image signal is modulated with reference to the time when the beam detector 5 detects the beam. That is, after a lapse of a predetermined time from a reference time, the laser beam starts to be modulated. In this manner, the image signal for each scanning line is aligned in terms of an output start position.
Provision of the proper control timings is needed for the following reason. In order to form an image on the photosensitive body after repeated scanning thereof by the laser beam, it is necessary to begin beam modulation at exactly the time at which each scanning action starts on the photosensitive body. Otherwise, the misalignment of the polygon mirror or the related parts thereof upon each scanning action may prevent that action from commencing at the proper position on the photosensitive drum. Without being properly timed, the resultant image can include rough, blurred edges.
The above drawback is circumvented by enhancing repeatability in two scanning directions. In the horizontal scanning direction, the modulation begins every time a beam detection signal comes in. In the vertical scanning direction, the reference clock signal is transmitted to the drum driving unit 61 so that the latter will drive the rotating drum 6 according to the timing of that signal. These measures are intended to expose the photosensitive paper to laser beam scanning with high accuracy and quality.
In the prior art image exposure apparatus, the semiconductor laser device is generally small-sized and has the advantage of being capable of direct modulation. However, the laser device is not without its share of disadvantages. One of such disadvantages is that, as depicted in FIG. 7, the intensity of the laser beam can vary significantly by the fluctuation in threshold current. The fluctuation is caused by changes in temperature and occurs over time even as the semiconductor laser device is being driven at the same current level. In addition to the varying beam intensity due to the temperature change and other factors, the semiconductor laser device is vulnerable to destruction caused by an excess current or excess output. At a low temperature in particular, the threshold current becomes small and thus tends to promote an excess output that often destroys the expensive semiconductor laser device.
The above disadvantage has been avoided by setting a low initial value of the beam intensity when the semiconductor laser device is started. Once the laser device is activated, the beam intensity is raised in steps up to a predetermined normal level for laser emission.
In order to implement high-quality exposure of high repeatability with the above prior art image exposure apparatus, it is conventionally necessary to control the rotating drum 6 with respect to the timing of beam detection by the beam detector 5. Specifically, a beam detection signal is sent to the drum driving unit 61 so that the latter will drive the rotating drum 6 according to the timing of that signal. This corrects the rotating misalignment of the rotating polygon mirror 41 and the like for control over scanning in the vertical direction.
Where the rotating drum 6 is rotated according to the timing of beam detection by the beam detector 5, the beam intensity is initially set to a low level so that the intensity will be raised later in steps, as mentioned above. This connotes that when the semiconductor laser device is started, the emission therefrom is so low that it cannot be detected by the beam detector 5. Therefore this image exposure apparatus cannot control the rotating drum 6 with respect to the timing of beam detection by the beam detector 5.
If the beam intensity is initially set to a level high enough for the beam detector to detect beam emission upon activation of the semiconductor laser device, the above-mentioned excess output is likely to occur, destroying the costly device.